1. Field of the Invention
The present invention relates to computer systems with virtual machines. More specifically, the present invention relates to a method and an apparatus that facilitates efficient linking of symbolic references in a virtual machine that supports in-place execution.
2. Related Art
The Java 2 Platform, Micro Edition (J2ME™), has become very popular in the wireless device space. Motorola, Nokia, NTT DoCoMo, RIM, Siemens, and many other key players in the wireless device industry have recently started shipping J2ME-enabled devices in very high volume. It has been estimated that over 200 million J2ME-enabled mobile phones were shipped in 2003 alone.
However, in spite of the success of the J2ME platform, significant challenges for Java™ technology remain in the memory-constrained device space. One major limiting factor in J2ME application development is application size. With the deployment of more significant J2ME applications in mobile devices, J2ME application downloads will consume an increasingly large amount of wireless network bandwidth. Moreover, the runtime dynamic memory costs and application startup costs associated with anything but small J2ME applications are still excessive for most mass-market consumer devices, as well as for serious mobile systems software development in the Java programming language.
Various techniques can be used to optimize the memory consumption and application startup costs of a Java Virtual Machine (JVM). One of the most promising approaches for dramatically reducing the memory consumption and application startup time of a JVM is a technology called “in-place execution.”
In a Java virtual machine that supports in-place execution, the standard Java class file format is replaced with a representation that can be used directly as runtime structures for the virtual machine, without the conventional and costly class loading process. Static data structures, such as classes, method tables, field tables, exception handlers, and symbols, are never actually loaded into the virtual machine. Instead, they are accessed directly from the static representation. In mobile devices, where the static representation can be kept in flash memory or ROM, this can result in dramatic RAM consumption savings, as well as battery conservation, typically on the order of five-fold to ten-fold. The static size of these applications is typically much smaller than with regular Java class files or JAR files. This can be accomplished without any loss of symbolic information or dynamic linkability of Java applications.
Unfortunately, in-place execution has disadvantages when it comes to execution speed. For example, since in-place executable code is immutable, various commonly used runtime optimizations that depend on the ability to modify bytecodes at runtime cannot be used. Moreover, since in-place executable files (henceforth referred to as “IPE files” or IPEFs) are intended to be portable and easily relocatable in memory, all the references in the files must be offsets or symbolic references rather than pointers. This introduces additional levels of indirection at runtime, which slows down execution speed.
Symbolic references (i.e., references that are “by name” rather than by pointer or offset) are especially problematic, since resolving a symbolic reference typically necessitates a costly symbolic lookup (e.g., a hashtable lookup using a string-based search key). Symbolic references are frequently needed in IPEFs, for instance, when referring to data structures or functions that are already assumed to be present on the target device when the IPEF is being installed on the target device.
Because of these problems, in-place execution has not yet been widely utilized in Java Virtual Machines, even though such technology would otherwise be ideal for mobile phones and other typical target devices of the Java 2 Platform, Micro Edition.
Hence, what is needed is a method and an apparatus that facilitates in-place execution without the problems listed above.